The exploitation of wind resources has been rapidly growing during the last years. However, the use of wind power provides not only energy, but also a negative impact to the power grid, to which it is connected. Not only changes in the wind but also the wind turbines themselves create power pulsations.
A solution is to use variable speed operation of the turbines by using transistor converters. The power quality can be improved, but at higher wind speeds the variable speed system has a lower efficiency compared to a fixed speed system.
To overcome such limitation converters may be used only at lower powers where there is an energy gain and use directly-connected induction generators at higher wind speeds as being disclosed in Power Quality Impact of a Sea Located Hybrid Wind Park, T. Thiringer, T. Petru, and C. Liljegren, IEEE Transactions on Energy Conversion, Vol. 16, No. 2, June 2001, p. 123.
Such a power converter arrangement for a wind turbine is shown in FIG. 1 during operation at lower and higher wind speeds. The arrangement includes two power converters 11a, 11b provided for accomplishing the variable speed operation at the lower wind speeds. At the higher wind speeds the constant speed operation is accomplished by connecting the induction generator 13 directly to the power grid 15. In this mode the power converter on the power grid side operates as a reactive power source with the purpose of providing a desired amount of reactive power and to reduce the voltage variations on the power grid caused by the active power variations from the wind turbine.
A similar power converter arrangement is disclosed in U.S. Pat. No. 7,012,409 B2 (SEMIKRON). A triggering method for power compensation in the event of a short circuit is also disclosed.